It is now very common to construct steam generating facilities having a furnace constructed of prefabricated tubular membrane panel walls These panel walls serve as a heat-absorbing surface and also provide a gas tight flow circuit for the generation of steam.
During the construction of the furnace, several panel wall units must be welded together to create the elongated furnace enclosure. Normally the tubes of these panel wall units are welded end-to-end, but sometimes a mid-region of a tube must be cut and re-welded for proper installation. No matter where the weld occurs, for safety's sake, the tube weld must be inspected to insure its compliance with a variety of guidelines, specifications, and codes. In particular, the critical portion of each tube weld (where the tube and the adjacent membrane bar are joined) must be inspected to prevent any possible failure under pressure.
The preferred inspection technique is radiography which creates an image of a number of tube welds simultaneously on radiographic film. This radiographic technique places the source of radiation some distance from the front of the panel wall with X-ray film being placed or taped on the other side of the panel wall. The radiation source is also vertically offset from the tube weld by about 15 to 20 degrees in order to separate the tube weld image of the near wall from that of the far wall on the X-ray film. Additionally, this method requires no cutting or removal of the membrane between adjacent tubes. Unfortunately, this technique cannot provide a clear image of the critical area of the tube weld because on the film the front critical area is superimposed over the back critical area, thereby distorting both images.
An improvement of this method is one where the source is not only vertically offset from the tube weld but is also offset to the right or left of the tube weld. Accordingly, the radiographic film must also be moved from its position adjacent to the back of the tube welds to a position along the side of each tube weld so as to remain generally perpendicular to the rays emanating from the offset source. This new position of the film necessitates the removal of a segment of the membrane bar between adjacent tubes. The additional cost incurred to remove and replace this membrane bar segment is considered well spent if the critical area of the tube weld can be inspected. Unfortunately, this is not the case because first, the offset of the source to the right or left cannot be too great or else the adjacent tube will block or interfere with the x-rays emanating from the source. Second, and precisely because of this lack of clearance with adjacent tubes, the front critical area of the tube weld will, on the film, still be superimposed over that of the back critical area of the tube weld. Another drawback is the fact that with this method, two 'shots' or views of each tube weld are required (one for the left critical area and one for the right) to obtain a full view of the tube weld; thus costs are doubled. However, this second method does provide better coverage of the critical area than does the first method; unfortunately, it is still a distorted view of the critical area and its coverage is still less than 100%.
It is thus an object of this invention to disclose an inspection system whereby 100% coverage of the tube weld is achieved including the critical area. It is also an object of this invention to describe a system whereby only one radiographic image need be taken to view a full tube weld. A further object is to eliminate any distortion of the image of the tube weld on the film. Another object is to provide an `actual size` image of the tube weld to aid in its inspection and/or investigation. Still a further object is to reduce the exposure time and the emissivity of the source required for each radiographic image thereby increasing the margin of safety for the personnel using such radioactive material. Yet another object of this invention is to reduce the time required to set up and complete each radiograph shot. These and other objects and advantages of the invention will become apparent upon further investigation.